The worldwide problem of processing waste materials concomitant a variety of industries is nowadays gaining in importance. In numerous cases such waste materials are known to represent carbonate suspensions. Thus, in sugar production they are available in the form of filtration sediments resulting from the production process, in the chemical technology in the form of chemical water purification of waste materials, and so forth.
Commonly such suspensions exhibit an initial water content of 38 to 55%, while the dry fraction thereof includes some quantities of calcium carbonate (CaCO.sub.3) (.about.80%), non-organic substances (approximately 80%) and organic substances (.about.12%). When processing them with a view to decompose the carbonate fraction, certain production processes involve a requirement concerning not only the derivation of highly active lime, but also the content of carbon dioxide in the waste gas so as to provide the possibility of utilizing this gas in the production process.
Thus, for example, the process of saccharine juice purification in the production of sugar makes use of lime and gas, the latter should comprise not less than 30% of carbon dioxide. What makes it difficult to obtain the gas of such a carbon dioxide content is that the suspension thermal treatment procedure following the conventional technology calls either for use of special-grade fuels containing an appreciable amount of carbon, such as coke and the like, or for use of specific facilities, such as electric calcining apparatus, or for use of oxygen blasting, since in the calcination, accomplished by means of ordinary apparatus employing liquid or gas fuel and air as an oxidizer, the content of carbon dioxide may reach theoretically only 27%. In actual production processes the content of carbon dioxide does not exceed 23-24%.
The thermal treatment using a solid fuel of a high carbon content is usually carried out in shaft furnaces, which significantly complicates the production process, inasmuch as separate suspension predrying and subsequent dry material briquetting procedures are made indispensible. In this case the lime has the form of briquets or cakes of considerable dimensions. It takes protracted periods of time to slake this lime, which extends the subsequent process of its slaking and deteriorates the quality of the calcareous suspension produced in the slaking.
Thus, it has become imperative to evolve the type of technology which would allow facile processing of a carbonate suspension in order to obtain lime and gas with a high content of carbon dioxide (30-35%) by means of an apparatus operating on liquid or gas fuel and air as an oxidizer, this being done at a minimum level of fuel consumption.
Along with rotating and multihearth furnaces, the present state of the art also makes extensive use of spray dryers and apparatus for the calcination of fluidized products enabling a fine-granulation product to be obtained. In this connection such apparatus can be contemplated as the most progressive for effecting the novel methods for treatment of carbonate suspensions.
Known in the art is a method of thermal treatment of a carbonate suspension in the form of a filtration sediment obtained from sugar production (see U.S.S.R. Inventor's Certificate No. 82010, Cl. 80 b 202, claims priority, May 10, 1948).
This method comprises the steps of predrying a filtration sediment, comminuting dried material and subsequently calcining the same to produce lime.
This prior art method is characterized by the fact that after comminution the dried material has a polydispersion granulometric composition, i.e. its composition includes both large pieces and a pulverized fraction as well. The calcination of this polydispersion material yields an ultimate product containing a substantial amount of burnt lime. This is attributable to the fact that the calcination of the small particles of the material at the same temperature takes a considerably shorter time for heating than the calcination of the large particles. As a result the calcination of the small particles with the same duration of heating at high temperatures results in burning. Thus, in order to improve the quality of the product the calcination should be preceded by sizing of particles. Furthermore, the fraction-by-fraction calcination of the particles of each and every size requires a number of separate units, one for each size. All this taken together makes the effectuation of the above method unnecessarily complex. In addition, the calcination of the polydispersion material is accompanied by considerable dust ejections, which brings about the contamination of the gas and increased expenditures for its treatment as well as losses of the finished material.
The prior art method is characterized by a fairly low carbon dioxide content in the waste gas and the possibility of controlling this content is not provided.
Known in the prior art is a method of calcining a calcareous slurry which is a waste of the kraft pulp activation process (see Japanese Pat. No. 48115, Cl. 15 F221, Int. Cl. COIF, published Apr. 12, 1972). This method comprises predrying a calcareous slurry of an initial water content of 40% to a final water content of 3%, mixing the same with 10% of granulated lime produced by means of a double-chamber briquetting press and granulated to a grain size of about 1 mm, and with 5-10% of limestone with an average grain diameter of about 3 mm, drying the resulting mixture to a water content of 2 to 5%, granulating the mixture by means of the press to a granule size of 35-50 mm and subsequently calcining the granules in a shaft furnace. It should be noted that in the foregoing method, the flow chart is fairly complicated due to the preparation of the mixture of the above three components prior to their granulation. Each of the components also undergoes a number of appropriate preparatory procedures, namely, the lime is ground, sized in a special separator and handled for separation of the particles of 1 mm in size, whereupon they are briquetted.
Furthermore, the calcination of the granulated briquetted mixture is carried out with the use of special kinds of fuel. Usually this is a fuel having a high carbon content such as of coke, the cost of which is higher than that liquid or gasiform fuel. In many cases this fuel is to be transported over substantial distances to the location of the shaft furnace, which also adds to the cost of fuel and makes the process more expensive. It is also noteworthy that the process of calcination in the shaft furnace is conducted for a long period of time and consequently is hardly amenable to control and automation.
The finished lime produced by this prior art method has the form of briquets or lumps which slowly hydrate in water thus increasing the duration of obtaining lime milk from the lime upon its further processing.
Known in the prior art is a method of producing lime and carbon dioxide-containing gas from a carbonate suspension which is a filtration sediment resulting from sugar production (see G.E. Kroneberger, Lime Recalcination in the U.S. Sugar Industry, Sugar Technology, Reviews, 4, 1976/77, pp. 3-47). The method under consideration comprises predrying a filtration sediment on the filter beds from a water content of 45% to a water content of 28% and further drying and calcining the same in a multihearth vertical calcinator. Drying of the carbonate suspension (i.e. filtration sediment) is accomplished by the gas discharged from the calcination area. The gas spent after drying and including carbon dioxide is passed for recovery to the process of lime milk carbonization. The above-described method is characterized by the fact that the waste gas after drying contains a fairly low amount of carbon dioxide (12 to 16%). Such a low content of carbon dioxide in the gas makes it practically unfeasible to use it in a sugar production process. Another particularity of the method consists in the fact that while propagating through the multihearth vertical calcinator, the product is continually mixed. Hence, the product is susceptible to excessive attrition and has a polydispersion make-up. This results in increased dust ejections from the multihearth vertical calcinator and in deteriorated finished lime due to the presence of particles of different degrees of dissociation. Moreover, the method shows a low intensity level of heat- and mass-transfer processes, since heat is supplied to the material in the heat-transfer process by way of thermal conduction, while thermal convection is actually absent. Therefore the treatment process is an objectionably protracted procedure, which handicaps control of the process due to the fact that a check on the properties of the material can be made only after it has gone through the entire process.
Also known in the prior art is a method of treatment of a carbonate suspension in the form of a sugar production filtration sediment yielding lime and carbon dioxide-containing gas by means of preliminary mechanical dehumidification of a carbonate suspension (i.e. filtration sediment) in a centrifuge (to a water content of 37%) followed by further drying and calcination of the carbonate suspension in a rotating kiln (see G. F. Kroneberger, Lime Recalcination in the U.S. Sugar Industry, Sugar Technology, Reviews, 4, 1976/77, pp. 3-47). This method is characterized by the fact that the spent gas leaving the rotating kiln includes a relatively small amount of carbon dioxide (14-16%), which encumbers its further use. Furthermore, high temperatures intrinsic in the waste (spent) gas (200.degree.-250.degree. C.) impairs the efficiency of the process. Additional operating costs are also incurred due to the fact that the rotating kiln has considerable overall dimensions (thus, for instance, a kiln whose throughput amounts to 1.200 kg/day is 2.9 m in diameter and 80 m in length), occupies a sizable portion of the production area and calls for substantial expenditures of time for its maintenance, startup and shutdown. The process conducted in the rotating kiln is a lengthy procedure (lasting a few hours), which makes process control a difficult matter.
Known in the prior art is a method of fabricating calcinated, pulverized lime from ground raw material or suspension, in particular from the waste materials of sugar production, which comprises drying the raw material and heating the same to a temperature lower than that of dissociation by a flow of the waste gas of the calcinator, calcinating the same in a gas flow and removing burnt lime from the calcination zone beyond the calcinator, and separating lime particles from the aforesaid gas flow (Cf. French Pat. No. 2,291,161, Cl. 04 B 1/02; C 13 D 3/02; F 27 B 1/100, publ. Feb. 6, 1976). In this method the material of the calcination zone moves in a direct flow with the gas flow, which interferes with the production of lime of high chemical activity on account of its potential interaction with carbon dioxide included in the gas, i.e. recarbonization (repeated combination of calcium oxide and carbon dioxide). Consequently, the content of carbon dioxide in the spent gas cannot be sufficiently high. The lowered content of carbon dioxide also stems from the large amount of air supplied to the calcinator since, according to the invention, the ratio of the excess air directed to burning amounts to 1.6 to 2.0. This also leads to a decreased temperature of the burning fuel, and a chemical and mechanical incompleteness of fuel combustion, which entails an increase in the consumption of fuel and reduces the efficiency of the method. The prior art method, apart from the above-mentioned, is also characterized the fact that the material produced from the suspension prior to its calcination, is not granulated. Owing to this, the material (lime) produced after its calcination is not subjected to uniform thermal treatment. The large particles of the material dissociate incompletely. The removal of the small particles of the material from the calcination zone necessitates a sophisticated process for their separation from the gas flow. All this involves much complexity in transportation and storage of the pulverized material. As a result, the operating costs tend to increase. Heating of the material to a temperature of 400.degree. to 800.degree. C. before its calcination may cause sublimation of the organic component and the appearance of noxious admixtures (carbon oxide and the like) in the spent gas.
Also known in the prior art is a method of recovering lime from calcareous sludge (Cf. U.S. Pat. No. 3,796,791, Cl. 423-177, publ. Dec. 3, 1974) which is most closely related in its technical essence to the subject of the present invention.
According to this method the calcareous sludge with a moisture content in the range from 40 to 60% is divided into two portions. One portion of the sludge is dried in a spray drier to a pulverized condition by the heat of the gas discharged from the calcination zone, whereafter the dried pulverized material is mixed with the other portion of the sludge to produce nonpulverous material with a moisture content in the range from 16 to 17%. This material is fed to a vertical multihearth calcinator and subjected to drying and heating to a temperature of 800.degree. to 950.degree. C. The carbonate fraction undergoes dissociation yielding lime--calcium oxide (CaO) and carbon dioxide (CO.sub.2). The resulting lime is cooled, while the gas from the calcinator is directed to the spray drier. The carbon dioxide-containing gas spent after drying is passed to utilization, i.e. to the process of saccharine juice purification.
The above-described prior art method is characterized by the fact that the spent gas directed to utilization includes a fairly low quantity of carbon dioxide (23.3%), which makes it difficult to use it in the manufacturing process since, for example, the gas directed to utilization in the production of sugar is prescribed by the technological requirements to contain not less than 30% of carbon dioxide. Another factor which makes the method lengthy and complex is the step of mixing the dry powder with the wet suspension. According to this prior art method, thermal treatment of the material introduced into the multihearth furnace does not enable the process of heat- and mass-transfer to be intensified since the convective heat-transfer portion in such a method of thermal treatment is not large.
With the above method of thermal treatment the calcinator burns down the total amount of fuel required both for the calcination of the material and for the drying of the carbonate suspension. As this takes place, heat losses from the calcinator to the atmosphere tend to grow, which may result in the present overconsumption of fuel. Moreover, the amount of carbon dioxide in the gas passed to utilization is not controlled.